The Prior Art
The addition of hydrogen sulfide (H.sub.2 S) to alkenes to produce alkyl mercaptans (Equation I) is well known. Solid catalysts such as alumina.sup.1, ferric oxide.sup.1, titania.sup.1, Fuller's earth.sup.1, silica.sup.1, silica-alumina.sup.1, chromia-alumina, acid clays.sup.1, phosphoric acid on Kieselguhr or carbon.sup.1, phosphotungstic or phosphomolybdic acid on alumina.sup.2, sodium or potassium phosphotungstate or phosphomolybdate on alumina.sup.2, and sodium or potassium tungstate on alumina.sup.3 can be employed in continuous, vapor-phase processes for mercaptan manufacture. The corresponding dialkyl sulfides, formed by Equations II and III, are generally obtained as by-products in these processes.
I. RCH.dbd.CH.sub.2 +H.sub.2 S.fwdarw.RCH(SH)CH.sub.3 PA1 II. RCH(SH)CH.sub.3 +RCH.dbd.CH.sub.2 .fwdarw.CH.sub.3 CH(R)SCH(R)CH.sub.3 PA1 III. 2RCH(SH)CH.sub.3 .fwdarw.CH.sub.3 CH(R)SCH(R)CH.sub.3 +H.sub.2 S R.dbd.H or alkyl FNT 1. E. Reid, Organic Chemistry of Bivalent Sulfur, Vol I P. 18, Chemical Publishing Co., Inc., New York, NY (1958) FNT 2. U.S. Pat. No. 3,036,133 FNT 3. U.S. Pat. No. 3,257,464
The amount of by-product dialkyl sulfide formed can generally be controlled by varying the molar ratio of hydrogen sulfide to alkene used in the reaction mixture. Higher molar ratios of hydrogen sulfide to alkene (e.g. H.sub.2 S/alkene=10-20/1) favor alkyl mercaptan formation, whereas lower hydrogen sulfide to alkene molar ratios (e.g., H.sub.2 S/alkene=1/1) favor dialkyl sulfide formation. The phenmenon is well illustrated in the drawing accompanying U.S. Pat. No. 3,036,133. Coventional catalysts, such as those cited above, have proven efficient for the production of alkyl mercaptans from alkenes and hydrogen sulfide, where a high H.sub.2 S/alkene molar ratio (above about 8-10/1) is employed in the reaction mixture. However, we have discovered that these conventional catalysts suffer serious shortcomings when they are used with low molar ratios of H.sub.2 S/alkene in the reaction mixture to produce predominantly the dialkyl sulfides.
The major shortcoming of the prior-art catalysts stems from the fact that, at the elevated temperatures needed for reaction to occur, combined with the low H.sub.2 S/alkene molar feed ratios required to favor sulfide formation over mercaptan formation, appreciable tar and coke formation occurs on the surface of the catalysts. The tar and coke clog the pores of the catalyst, render its reaction-sites inaccessible to the reactants, and deactivate the catalyst after a relatively short period of operation in a continuous, vapor-phase process. The addition of an inert diluent, such as nitrogen or methane, to the feed mixture to remove heat from the reaction zone and eliminate hot spots in the catalyst bed fails to prevent the tar and coke formation. To sustain a high production rate of dialkyl sulfides with these conventional catalysts, it is necessary to incorporate an air-regeneration cycle into the process to remove the accumulated tar and coke from the surface of the catalyst periodically and thereby restore high catalyst activity. This requirement adds appreciably to the cost of the process.
Diethyl sulfide (DES) is the preferred product of the process of this invention. DES is a well known article of commerce, being used in gas odorant mixtures and as a sulfiding agent for the post-regeneration treatment of hydrodesulfurization catalysts in petroleum refining. In the past this material has been available as a by-product from the manufacture of ethyl mercaptan. Recently, however, the demand of DES has expanded to the extent that there is a need for an efficient process by which it can be manufactured independently.